Mobile products such as personal computers, digital cameras, and mobile phones are growing in popularity in recent years, and are desired to become more compact and more functional.
For example, recent mobile phones are required to have the function of taking pictures at a high-definition resolution of 2 million pixels or more.
Patent Document 1 discloses a module with built-in components and a camera module that each include a multilayer circuit board. Such modules, which are used on another circuit board like a motherboard, are expensive, and therefore, it is required to use inexpensive circuit boards.
FIG. 11 is a sectional view of a mounting body in which the camera module of Patent Document 1 including a conventional multilayer circuit board is mounted on a multilayer printed wiring board. In other words, FIG. 11 shows a sectional view of a mounting body for a camera module in which multilayer printed wiring board P2 for the camera module is mounted on multilayer printed wiring board P1. Multilayer printed wiring board P1 can be a laminated body of a glass fiber and an epoxy resin, and can be a motherboard for a mobile phone. Multilayer printed wiring board P2 can be a multilayer printed wiring board of several millimeters square for a camera module. Multilayer printed wiring board P2 may be designed for a module with built-in components.
As shown in FIG. 11, large multilayer printed wiring board P1 and small multilayer printed wiring board P2 are each composed of a plurality of layers of wiring 1 stacked through insulating layers 2. Wiring 1 is interconnected through vias 3. Large multilayer printed wiring board P1 and small multilayer printed wiring board P2 are connected via solder balls (not shown).
On the surface of multilayer printed wiring board P2 for the camera module, there is fixed image pickup device 5 such as a CCD in such a manner as to be surrounded by holder 4. Image pickup device 5 is connected to wiring 1 of multilayer printed wiring board P2 through wires 2a and other devices. On holder 4, there is fixed lens 6 having optical axis 7. As shown by the arrow, light 7a from outside projects a predetermined image on image pickup device 5 through lens 6. Holder 4 supports auto-focusing.
In a mounting body including a multilayer printed wiring board, such as a mounting body for a camera module or for a module with built-in components, however, multilayer printed wiring boards P1 and P2 may be subjected to warpage. The warpage is due, for example, to heating and cooling in a solder reflow process during manufacture, or to temperature changes in the usage environment. FIG. 12 is a sectional view showing the warpage problem of multilayer printed wiring boards P1 and P2. As shown in FIG. 12, multilayer printed wiring boards P1 and P2 tend to have warpage 8 by their difference in material or the coefficient of thermal expansion. Warpage 8 means poor flatness, and includes undulation and distortion. As shown in FIG. 12, warpage 8 may result in peeling 9a at joints between large multilayer printed wiring board P1 and small multilayer printed wiring board P2, for example, at joints in a BGA (Ball Grid Array) package having solder balls. In addition, small multilayer printed wiring board P2 and holder 4 may have gap 9b therebetween, which affects the optical system including lens 6. As a result, optical axis 7 of lens 6 deviates as shown by optical axis 7b, making lens 6 out of focus.
Warpage 8 and peeling 9a may also occur in the case of using a module with built-in components instead of a camera module, causing improper connection of wires or other problems.
As described above, the conventional multilayer printed wiring board and the mounting body for a camera module using the board have problems such as optical axis deviation and improper connection due to the warpage and twist of the multilayer printed wiring board itself.
Patent Document 1; Japanese Patent Unexamined Publication No. 2007-165460